Thank you for the kind words. Glad to hear that you chose to register DiffMerge.

When you registered the product, you should have received a registration key. Did you receive a key via email? If so, did you install it on your machine? When you randomly receive the registration notification are you by any chance using DiffMerge on a different machine?

Yes, I've entered the registration key and it is displayed as "valid". I think I've entered the key on three different machines (work, laptop, home), but mostly only use one of those at a time. Yesterday I had the dialog pop up several times (hence the report) and at that time, I definitely only had my work machine running.

Your DiffMerge registration is not device specific, so you can install the registration key on multiple machines (which it sounds like you've already done?). Can you please confirm you've installed the registration key on each device? You should also be able to run each machine simultaneously. Could you please email me a screenshot of your registration key on the device that is giving you problems? By selecting Tools, Registration. You can email it to "support@sourcegear.com".